#!/usr/bin/python

flag = 1
while flag:
    a = input("输入一个0-100的数字:")
    if not a.isdigit():
        print("请输入正确分数")
    elif 0 <= int(a) < 70:
        print("D")
        flag = 0
    elif 69 < int(a) < 80:
        print("C")
    elif 79 < int(a) < 90:
        print("B")
    elif 89 < int(a) < 101:
        print("A")
    else:
        print("请输入0-100分数")
